An advisory committee of the U.S. space agency NASA has issued a warning regarding the way Elon Musk-led space firm SpaceX intends to fuel rockets that will eventually carry astronauts.

SpaceX has plans to ferry humans between Earth and the International Space Station (ISS) and eventually to Mars on its Crew Dragon spacecraft. But, the company’s plans to fuel the spacecraft while humans are aboard have raised concerns.

Pointing out that no company has ever been near the launch pad when a rocket was fueled, the International Space Station Advisory Committee said that the proposal is risky. Committee Chair Thomas Stafford called the proposed fueling plan a “hazardous” operation.

John Logsdon, professor emeritus at George Washington University’s Space Policy Institute, “I’m not aware that in any other U.S. human spaceflight launch, the booster is fueled after the crew is aboard. It’s a deviation from the norm, and that’s bound to raise concerns.”

But, the company officials say that its spacecraft must be fueled just before launch so that the fuel doesn’t warm up. It means astronauts will likely be aboard ahead of fueling in the future, when the company will be ready to ferry humans to the ISS.